In the manufacture of microelectronic devices, various microelectronic components are electrically joined together into circuits using metal alloy solders. Such solders are generally alloys of tin (Sn) and lead (Pb), but can comprise other metal components. The metal alloy solder is generally optimized for bulk properties that control the thermal behavior of the solder. For example, the melting temperature of Sn/Pb solder is controlled by adjusting the relative amounts of tin and lead in the solder. The minimum melting temperature of a Sn/Pb alloy is achieved when the ratio by weight of Sn to Pb in the solder is 63:37. This is called the eutectic composition.
A solder that is homogenous across its mass and optimized for bulk properties does not take into account the desirable surface properties of the solder. Therefore, solder compositions that are optimized for bulk behavior are not necessarily optimized in terms of surface properties. Such surface properties control wetting to metal surfaces, reactivity with organic materials such as fluxes, and the melting point of the surface layer. Such surface properties depend upon both the chemical composition of the solder and the physical characteristics of the surface. For instance, a rough surface may have more surface area for holding flux than a smooth surface.
For some applications, it is desirable to use bulk solder alloys at other than the eutectic composition. For example, Controlled Collapse Chip Connection (C4) solder balls, which are used to join Integrated Circuit (IC) chips to chip carriers, are typically Pb-rich in the range of 90% to 97% Pb. Because Pb-enriched solders have higher melting temperatures, the solder joint between the chip and the chip carrier maintains its shape throughout subsequent assembly of the chip carrier (with chip attached) to another electrical component such as a printed circuit board. The Pb-enriched solders also require higher temperatures, however, to fuse them to metal surfaces such as copper pads on chip carriers. Thus, organic laminate chip carriers having copper joining pads may be exposed to undesirable high temperatures during joining processes.
A tradeoff results: without further treatment, the benefit of using a Pb-enriched metal alloy solder that maintains its shape at high temperatures is offset by the need for a high fusing temperature. Such high fusing temperatures can be avoided by applying a eutectic solder to the joining pads on the organic laminate chip carrier. To ensure proper fusing at every joint, however, the height of the applied eutectic solder layer must be controlled within very narrow limits, a requirement that is difficult to satisfy.
Another modification that retains high temperature stability while providing low temperature fusing comprises placement of an additional layer or cap of tin or other low melting point metal on the surface of the Pb-enriched solder, as described in U.S. Pat. No. 5,634,268 and U.S. Pat. No. 5,729,896 and assigned to the common assignee of the present invention. Such a cap enables the bulk solder to retain its desirable high-temperature resistance while providing a surface layer capable of fusing at low temperatures.
The present invention proposes an alternative process for providing a solder surface layer capable of fusing at low temperatures while maintaining the bulk solder properties. It is an object of the present invention to provide a process for altering the chemical composition and physical characteristics of a metal alloy solder surface without adding an additional metal layer. It is a further object of the present invention to provide a process for altering the surface characteristics of a metal alloy solder simultaneously with roughening the surface of the substrate to which the solder is connected.